This is something easy to do and Google highly recommends you to do it. It is always important that what Google likes you should do if possible.
File names are the names of your website pages. For example: A page about dog food you probably will name it something like dogfood.html
Google suggests you use punctuation marks in your file names. Instead of dogfood.html put it dog-food.html
You should use the – symbol to separate words, not the _ symbol.
This is great way for visitors to understand faster and easier what the page is about. Makes sense.
Google always tried to give their users (the people who search the search engine) the best results and experience. These little things make a difference, and for search engine optimization purposes you will get (even though small) credit. It is a good thing to do.
Do not however go and rename all your website pages. That will results in lots of 404 page not found error pages. Just for next time when you create pages, start using the punctuation symbol to separate words.
Let me give you few more examples. Wrong way to name a page is catfood.html or cheapcatfoods.html
Good way to name a page instead is cat-food.html or cheap-cat-foods.html Easy and simple tip. File names also make more sense.
Something to think about now. Blogger blog platform is created and owned by Google. By default the blogs you created are search engine optimized. Notice the file names of the post pages. They are all created with the punctuation symbol. Google implemented this in the blogger blogs by default.
Here are some more tips on this subject.
1) Do not create a ton of different folders or directories on your website. For example create a folder named articles and put all articles inside it.
2) Avoid creating sub domains with identical or very similar content as your domain name.
3) Make meta tags unique for each webpage. If your site is big, you obviously cannot make each page with a unique meta tag. In that case at least make the most important pages with very descriptive meta tags that clearly describe what the page is about.
4) Avoid also creating pages like this: www.domainname.com/Support
(You do that by creating a new folder called Support, then putting an index.html page inside the folder) That can result in some errors too with redirection and so on.
So it is best to create html pages, like this: www.domainname.com/Support.html
These things that deal with how to structure your website and so on are all known as onpage optimization strategies/methods or techniques.
They are not so important as the offpage optimization strategies, which deal with getting links. But they still count and obviously simple things like these you should start implementing them.